Presently, there is a wide distribution of products that use image compression encoding technology such as video cameras and digital versatile disks (DVDs). Moreover, there are active discussions in the field of image compression encoding to further improve the efficiency and quality of compression encoding with regard to the next generation of compression encoding technology.
For example, Moving Picture Experts Group-2 (MPEG-2) and H.264 MPEG-4 advanced video coding (AVC) (hereinafter may be referred to simply as “H.264”) are known as standards established by the International Organization for Standardization/International Electrotechnical Commission (ISO/IEC) with regard to compression encoding technology.
Motion compensation is performed when encoding between frames in compression encoding such as H.264. Motion compensation is a method for detecting, for example, how a subject or the like moves before and after successive images (or image frames, herein after may be referred to simply as “frames”). Specifically, motion compensation is a technique, for example, that involves an image compression device searching for motion vectors based on a decoded image (or a reference image) and an image to be encoded, generating a predictive image in which the reference image is shifted by the motion vector, and performing compression encoding on the differential between the generated predictive image and the image to be encoded. It is possible to perform compression encoding that uses the correlations between frames to greatly compress the information amount of image data by using predictive encoding based on motion compensation.
However, with images taken using operations such as zooming and the like in cameras, a differential value that is too large to be taken as a differential may be generated in each pixel before and after the frame even though the fact that the size of a certain subject is made larger and smaller can be recognized by human eyes before and after a frame. In this case, even if motion vector searching is performed by using motion compensation, the motion vector is difficult to be calculated accurately. Further, even if a predictive image is generated, the predictive image and the image to be encoded differ greatly and consequently the compression rate is reduced.
Accordingly, there is a technique called global motion compensation. Global motion compensation is a technique for generating a reference image for handling, for example, zooming, panning, or tilting or the like of a camera. It is possible to process a reference image by using the global motion compensation and improve accuracy and avoid a reduction in the compression rate in comparison to the abovementioned motion vector. However, although global motion compensation has been applied to MPEG-4 version 2 (V2), the use of global motion compensation in H.264 has been ignored.
For example, a known technique that is related to compression encoding may be provided as an encoding device that increases or reduces the size of a reference image pertaining to an inter-encoding image, uses a reference image the size of which has been increased or reduced to generate a predictive image, and uses the generated predictive image to encode an image to be encoded.
According to this technique, the encoding efficiency may be improved up to a certain extent when increasing or reducing the size of the image to be encoded.
Examples of the related art include International Publication Pamphlet No. WO 2015/098559.